In view of the increasing importance of climate protection and CO2 emissions, the production of insulating materials has developed into an important branch of economy in recent decades. For example, an improved insulation of buildings would have the consequence that less thermal output, which accounts for about 40% of the energy consumption in Germany, would have to be produced, and thus CO2 emissions would be reduced (Lünser, H., Dämmstoffe im Hochbau (2000), Wirtschaftsministerium Baden-Württemberg, Stuttgart). However, for these insulating materials to be employed in an economically reasonable way, the cost and expenditure of their production are critical factors.
In recent years, cellular materials or foams have proven to be particularly effective insulating materials. For such foams, the thermal conductivity λ (in Wm−1 K−1) is a critical factor. It expresses the capability of a material to transmit heat or to insulate. The thermal conductivity is composed of three factors (FIG. 1). These contributions are heat convection through the cell gas included in the pores of the foam, heat conduction through the web material (cellular matrix), and heat radiation (infrared radiation). FIG. 2 shows the different contributions to the overall thermal conductivity of a polystyrene foam (EPS). As can be seen, the cell gas makes the highest contribution in terms of heat convection.
A possible approach to reducing heat convection through the cell gas is the reduction of pore size, since the so-called Knudsen effect gains importance below a particular pore size. It implies that for gas molecules that are in a closed space whose diameter is smaller than twice the mean free path of the gas λG, the probability of collision with a wall is higher than that of collision with another gas molecule. Thus, a directed movement of the gas within the pores is no longer possible, and therefore heat convection by the gas breaks down in this limiting case (Seinfeld, J. H. and Pandis, S. N., Atmospheric Chemistry and Physics (1998), Wiley-Interscience, New York; Raed, K. and Gross, U., International Journal of Thermophysics 4: 1343-1356 (2009)). The mean free path for air at room temperature is λG=70 nm. Therefore, in order to utilize the Knudsen effect in insulation materials, pore sizes of below 140 nm would have to be realized. As compared to conventional foams, the thermal conductivity of such a nanocellular foam would be significantly lower, which is why it would be possible to work with significantly thinner insulation layers. This in turn would lead to a considerable saving of raw materials.
Several methods have been known to date for producing such nanostructured foams, the two most promising approaches being the sol-gel process used in aerogel production, and the principle of supercritical microemulsion expansion (POSME; Kistler, S., Journal of Chemical Physics 36: 52-64 (1932); Strey, R. et al., DE 10260815B4). An advantage of the sol-gel process is the fact that supercritical drying of the fixed gel is necessary in the last process step, which is why this method proves to require a high expenditure and thus to be cost-intensive. In comparison, the POSME method is considerably more cost-effective. In this method, a supercritical microemulsion characterized by a structure size of 1-100 nm is used as a template. In this way, the production of nanoporous materials should also be possible by fixing the microstructure and at the same time continuously expanding the microemulsion. However, it has not been possible to date to transfer the structure of the microemulsion to the foam without coarsening, because ageing phenomena occur during the fixing process, which coarsen the structure (Khazova, E., Doctoral Thesis (2010), Cologne University). Further, a surfactant is necessary for the thermodynamic stability of the microemulsion, which contaminates the product on the one hand and adds to the cost on the other. Therefore, presently, both the sol-gel process and the POSME method are unsuitable for industrial-scale applications.
Another approach for producing nanoporous materials could result from the use of polymer nanoparticle dispersions. By depositing the polymer from the corresponding dispersion, it is possible to produce amorphous packings of nanoparticles or nanoparticle crystals in which the polymer particles are in close packing. Through the action of a supercritical gas on such a packing of nanoparticles while heating is performed above the glass transition temperature of the polymer, gas can be entrapped in the gaps of the packing. Thus, nanodisperse inclusions would be formed in the polymer matrix. These inclusions can be foamed by subsequent expansion, whereby the production of nanoporous materials should be possible. As compared to the POSME method, this process has the advantage of dispensing with the use of surfactants. Further, thin nanoparticle layers, which can be subsequently foamed, can be applied to surfaces without difficulty by controlled deposition processes.
The object of the present invention is to show that the production of nanostructures polymers or foams is possible through the generation of nanodisperse inclusions in a high viscosity matrix.